This invention relates generally to aircraft antenna systems and, more particularly, to aircraft antenna systems that conform to the surface of the aircraft and electromagnetically excites at least adjacent portions of the aircraft structure. U.S. patent application Ser. No. 08/712,686, filed Sep. 12, 1996 and entitled "Multifunction Structurally Integrated VHF-UHF Aircraft Antenna System," now U.S. Pat. No. 5,825,332, discloses an aircraft antenna system structurally integrated into an aircraft tail fin. Basically, a notch antenna is incorporated into an endcap structure of the vertically oriented tail fin assembly and excites a vertically-polarized electric field.
There are some specific applications of airborne electronics systems in which specialized electronics equipment is housed in an external pod attached to an aircraft. For example, synthetic aperture radar (SAR) equipment is typically housed in a pod mounted beneath an aircraft. The pod provides a protective, yet radio-frequency-transparent, housing for the SAR equipment, which performs radar scanning of the topology beneath the aircraft. Moreover, housing such equipment in a removable pod facilitates maintenance of the equipment and allows it to be moved from one aircraft to another with less difficulty.
A requirement of some of these applications is that there must be a capability to handle multiple broadcast and reception of radio-frequency (RF) signals sharing the same frequency band with a minimum of system degradation. This necessarily entails the separation of transmit and receive functions as much as possible. There are various techniques for meeting this requirement, such as phase cancellation, frequency separation, and spatial separation of separate transmit and receive antennas. One of the objects of the present invention is to provide separate transmit and receive antennas on aircraft, especially aircraft that have external equipment pods.
Although the prior patent application referred to discloses an antenna system that provides good performance for very-high-frequency (VHF) and ultra-high-frequency (UHF) radio signals, there is still a need for an antenna system that produces both vertically polarized and horizontally polarized fields, and that satisfies the requirements discussed above.
U.S. Pat. No. 5,184,141 to Connolly et al. suggests integration of an antenna into a load-bearing member of an aircraft structure. However, the antenna in Connolly et al. is a dipole or other type of antenna installed behind a transparent window in the aircraft surface, and does not directly excite any portion of the aircraft structure.
Accordingly, there is still a need for a multifunction antenna system for installation in manned or unmanned aircraft, to satisfy the requirement for spatially separated transmit and receive antennas. The antenna system should provide omnidirectional patterns of both vertically polarized and horizontally polarized radiation and should have low cost and weight penalties. The present invention meets all these requirements and has additional advantages over the prior art.